Using range maps to plan protected areas

Trade-offs in the use of species distribution maps for protected area planning

Key messages:

Planning for new protected areas using range maps can lead to overestimating the level of protection, due to commission errors

The adoption of a coarse analytical resolution can slightly mitigate this effect but leads to inefficient planning

Intermediate resolutions are the best compromise to reduce commission errors while maintaining efficient planning

From the local to the global scale, conservation decisions are heavily influenced by the knowledge of where species are found. Maps of the geographic range of species (or simply ‘range maps’) are typically used to determine the overlap between threatened species and protected areas, and to find new places in need of protection.

However, range maps are usually incomplete and often contain errors. Commission errors are where species are supposed to be present in locations where they are actually absent, and omission errors describe the opposite situation, where species are mapped as being absent when in fact they are present.

Commission errors are particularly worrisome in conservation because they can lead to a false perception of species protection – ie, that a species is better protected than it actually is. They can also steer conservation investment toward areas of little conservation value, where species are not present.

Habitat suitability models can be used to reduce the effect of commission errors by removing from a range map those areas which are considered unsuitable for the species. However, habitat models are data demanding and their use is not always possible, especially for analyses focused on many species.

Another method for reducing commission errors is by using a coarser analytical resolution. For example, if a range map uses coarse grids (100 – 200 km squares), the probability of including unoccupied grid cells is reduced. Commission errors are averaged out. Unfortunately, the adoption of a coarse resolution also affects the efficiency of a conservation plan (ie, the ability to select a minimal additional area to be protected for achieving an adequate representation of all species).

While the problem of commission errors in range maps has long been known, the size of the trade-offs (ie, what is lost and gained through the use of coarser analytical resolution), has never been quantitatively explored. We set out to fill this hole by performing a set of analyses comparing protected-area planning for the world’s threatened terrestrial mammals at various resolutions. We compared species range maps with habitat models to show the difference between protected species ranges and protected habitats (Di Marco et al, 2017).

Spatial distribution of the fossa (Cryptoprocta ferox), a threatened mammal (a cat-like, carnivore closely related to the mongoose) endemic to Madagascar (pictured on the right, image by Chad Teer, CC BY 2.0). Panel (a) shows the global location of the species range. Panels (b–g) show the proportion of species geographic range within grid cells at various resolutions (from 10 km to 200 km). Panel (h) shows a binary reclassification (presence/ absence) of the species range at a 100 km resolution; in this case a cell was considered to be entirely occupied if 5% or more of its area overlapped with the species range, and entirely unoccupied otherwise.

Our analysis involved a global conservation planning analysis. We began by using range maps from the IUCN for the world’s 1,115 species of threatened terrestrial mammals. When employing a resolution of 10 km per map square, a global protected area expansion of 3 million km2 (an area almost the size of India) would suffice to achieve adequate protection for all the species.

However, if you used habitat models to determine what parts of that designated extra protected area was actually unsuitable for the species it was supposed to protect, you find a shortfall of 28 species (ie, species that appear to be adequately protected by their ranges, but not by their habitats).

At a coarser resolution of 200 km (per map grid), the shortfall for an equal figure of protected area expansion would be just 7 species. At this coarse resolution it was also twice as likely (80% vs 40% at a 10 km resolution) that the priority grids for the protection of species ranges were also considered a priority for protecting species suitable habitats. However the adoption of a 200 km resolution lead to the selection of a total of 12 million km2 of protected area in order to achieve adequate coverage for all species, which is four times larger than the area selected under a 10 km resolution.

We believe these findings demonstrate that adopting coarse resolutions in protected area planning results in an unsustainable increase in costs, with limited reduction in the effect of commission errors in IUCN range maps. Given this, we recommend that, if some level of uncertainty is acceptable to managers, using range maps at resolutions of 20–30 km is the best compromise for reducing the effect of commission errors while maintaining cost-efficiency in protected area planning analyses.

“Protected Areas have been the ‘big idea’ of biodiversity conservation over the last one hundred years. The total area and the number of protected areas have increased dramatically from a handful in the 1900s to over 160 thousand covering over 28 million square km today. However, they still only cover about 5.6% of the earth’s surface which is not sufficient to slow down the extinction crisis.

Setting up new protected areas is a challenging task. One of the important aspects of the process is to minimize costs of effectively protecting an area with choosing an area to maximise the biodiversity inside. Di Marco et al (2017) have made a valuable contribution to this optimisation problem. This paper is a fine example where a global-scale analysis has very practical value for site-specific conservation challenges such as protected area planning. Di Marco et al also pave the way for future work to analyse the trade-offs for less-studied groups of species such as amphibians, reptiles and insects.”